1. Field of the Invention
The present invention relates to color image reading apparatus and, more specifically, to color image reading apparatus, suitably applicable, for example, to color scanners, color facsimile devices, and the like, that can precisely read color image information on an original surface without lowering the reading resolution in the sub scanning direction by use of color separation means comprised of a reflection type or transmission type one-dimensional blazed diffraction grating, light receiving means comprised of plural (three) line sensors (light receiving elements) disposed on a common substrate surface, and a diffraction optical element.
2. Related Background Art
A variety of proposals have been presented heretofore on apparatus for forming an image of color image information on the original surface, on a surface of line sensor (CCD) through an optical system and digitally reading it by making u se of output signals from the line sensor at this time.
For example, FIG. 1 is a schematic diagram to show the major part of the optical system of a conventional color image reading apparatus. In the same figure, when a beam from a color image on original surface 81 is condensed by imaging lens 89 to be focused on a surface of each line sensor described below, the beam is color-separated into three colors, for example red (R), green (G), and blue (B), by 3P prism 80 and thereafter each beam is guided onto the surface of each line sensor 81, 82, 83. The color image formed on the surface of each line sensor 81, 82, 83 is read every color light by line scanning in the sub scanning direction.
FIG. 2 is a schematic diagram to show the major part of the optical system of another conventional color image reading apparatus. In the same drawing, when the beam from the color image on original surface 81 is condensed by imaging lens 89 to be focused on the line sensor surface described below, the beam is separated into three beams corresponding to the three colors by two beam splitters 90, 91 for color separation provided with wavelength-selective transmission films having dichroism.
Then the color images based on the three color beams are focused on the surface of so-called monolithic 3-line sensor 92 in which three line sensors are disposed on a common substrate surface. Then the color images thus formed are read every color beam by, line scanning in the sub scanning direction.
FIG. 3 is an explanatory drawing of the monolithic 3-line sensor 92 shown in FIG. 2. The monolithic 3-line sensor 92 has three line sensors (CCDs) 95, 96, 97 a finite distance apart from each other and in parallel on the common substrate surface as illustrated in the same figure, and a color filter, not illustrated, based on each color beam is provided on the surface of each line sensor 95, 96, 97.
Spaces S1, S2 between the line sensors 95, 96, 97 are usually made to be, for example, about 0.1 to 0.2 mm from various manufacturing conditions, and pixel widths W1, W2 of each single element 98 are set to, for example, approximately 7 .mu.m.times.7 .mu.m or 10 .mu.m.times.10 .mu.m.
The color image reading apparatus shown in FIG. 1 needs the three independent line sensors, is required to have high accuracy, and, in addition, needs the 3P prism which is not easy to manufacture. Therefore, the overall apparatus becomes complicated and expensive. Further, it necessitates three independent alignment adjustments between the imaging beams and the respective line sensors. Therefore, the apparatus had the problem that the assembly adjustments became troublesome.
In the color image reading apparatus shown in FIG. 2, when the thickness of wavelength selective transmission film of the beam splitter 90, 91 is x, the distance between the lines of line sensor is 2.sqroot.2x. Supposing the distance between the lines of a line sensor preferred in respect of manufacturing is approximately 0.1 to 0.2 mm, the thickness x of wavelength selective transmission film of the beam splitter 90, 91 is approximately 35 to 70 .mu.m.
In general, it is very difficult to construct the beam splitters in such small thicknesses while well maintaining optical planeness. Use of the beam splitters in such thicknesses raised the problem of degradation of optical performance of the color images formed on the line sensor surface.
On the other hand, the distances S1, S2 between the center line 96 and the other two lines 95, 97 of the monolithic 3-line sensor are usually set to be equal distances in the opposite directions to each other and to be equal to an integral multiple of the pixel size W2 (see FIG. 3) in the sub scanning direction as shown in FIG. 4. This is from the following reason.
When the color image is read by the above-stated monolithic 3-line sensor by use of only the ordinary imaging optical system 89 as shown in FIG. 4, reading positions on the original surface 81 that can be read simultaneously by the three line sensors 95, 96, 97 are three different positions 95', 96', 97' as illustrated.
Owing to this, signal components of the three colors (R, G, B) for an arbitrary position on the original surface 81 cannot be read at one time, so that signals must be matched and combined for each position after read by the 3-line sensor.
Synthetic signal components of the three colors are obtained relatively easily, by setting the distances S1, S2 between the lines of the 3-line sensor to the integral multiple of each pixel size W2, providing the apparatus with redundant line memories corresponding thereto, and delaying G, R signals (signal components based on the G and R color beams) with respect to B signal (a signal component based on the B color beam), for example.
Accordingly, the distances S1, S2 between the center line sensor 96 and the other two line sensors 95, 97 of the 3-line sensor are set to the integral multiple of the pixel size W2 in the sub scanning direction as described above.
However, provision of the redundant line memories corresponding to the distances between the lines of 3-line sensor in the above-stated color image reading apparatus posed problems that the apparatus must be provided with a plurality of expensive line memories, this was very disadvantageous in terms of cost, and the overall apparatus became complicated.
In addition to the above examples, another color image reading apparatus for making the color image information color-separated by a one-dimensional blazed diffraction grating as an optical element for color separation and incident to the monolithic 3-line sensor and for detecting the color image information thereby was proposed in U.S. Pat. No. 5,223,703, for example.
FIGS. 5A and 5B are drawings to show the color image reading apparatus disclosed in above U.S. Pat. No. 5,223,703. FIGS. 5A and 5B are drawings to show the arrangement in the sub scanning cross section perpendicular to the main scanning cross section. In the drawing, the image information on the original surface 101 being an object is line-scanned in the sub scanning direction (vertically on the plane of FIG. 5A) by a mirror (not illustrated) etc. disposed between the original surface and the imaging optical system 102, and then the image information light is guided through the imaging optical system 102 to reflection type one-dimensional blazed diffraction grating 103 for separation into the three colors.
Here, information light from a same position (a same line) of the original surface 101 is separated horizontally in the drawing by reflection diffraction into beams 105, 106, 107 of the three colors (for example, R, G, B) in the so-called color reading, and thereafter the beams are focused on respective sensor arrays, i.e., line sensors 108, 109, 110 on the monolithic 3-line sensor 104.
Then the image information of the original surface 101 is successively read by relative movement between the original surface 101 and the image reading apparatus (imaging optical system 102, diffraction grating 103, sensor 104) in the sub scanning direction.
Here, each sensor array 108, 109, 110 on the sensor 104 extends in the main scanning direction normal to the plane of the drawing. The sensor 104 is a monolithic 3-line sensor in which three lines of one-dimensional sensor arrays are arranged a finite distance apart from one another in a direction perpendicular to the array direction on the same substrate.
The one-dimensional blazed diffraction grating 103 is disposed in an optical path between the imaging optical system 102 and the sensor 104 and on the sensor 104 side of the exit pupil of the imaging optical system 102 and is provided for color-separating light from the object into plural beams and guiding the color-separated beams to the respective sensor arrays corresponding thereto.
The original surface 101 is illuminated by a light source for illumination not illustrated and the image information thereof is read by the image reading apparatus.
The above one-dimensional blazed diffraction grating for three-color separation is described in Applied Optics, Vol. 7, No. 15, pp 2273-2279 (Aug. 1, 1978) and the configuration thereof is as shown in the enlarged view of FIG. 5B, which is a drawing to show the configuration in the sub scanning cross section.
Incidentally, distances 116, 115 on the sensor surface 104 between separate images of .+-. first-order diffracted beams 107, 105 and zero-order beam 106 separated by reflection diffraction by the reflection type one-dimensional blazed diffraction grating 103 shown in FIG. 5B are expressed by the following equation, letting Z be the distances and using the symbols in FIG. 5B. EQU Z=l.times.tan {sin.sup.-1 (.+-..lambda./p+sin .theta..sub.0)-.theta..sub.0 }(1)
(where .lambda. is the wavelength of the information light separately imaged, .theta..sub.0 an angle of incidence to the blazed diffraction grating 103, p the grating pitch, and l a distance on the optic axis between the grating and the light receiving surface).
For example, supposing in the step configuration of the reflection type blazed diffraction grating 103 the depth h.sub.10 of the first step and the depth h.sub.20 of the second step are h.sub.10 =909 nm and h.sub.20 =1818 nm, respectively, the center wavelength of the zero-order beam is .lambda..sub.0 =525 nm, that of the + first-order diffracted light is .lambda..sub.+1 =592 nm, and that of the - first-order diffracted light is .lambda..sub.-1 =472 nm. This is based on the following equation. EQU .lambda.=2h.sub.10 .multidot.cos .theta..sub.0 /m=2h.sub.20 cos .theta./2m(2)
Here, .lambda..sub.+1 is obtained by m=3-1/3, .lambda..sub.-1 by m=3+1/3, and .lambda..sub.0 by m=3 (though the values obtained are approximate values as to .lambda..sub..+-.1).
When the grating pitch of the above diffraction grating 103 is set to p=130 .mu.m, the distance on the optic axis between the grating and the light receiving surface to l=45 mm, and the angle of incidence to .theta..sub.0 =30.degree.,
Z.lambda..sub.+1 =0.171 mm, and PA1 Z.lambda..sub.-1 =0.136 mm. PA1 in that a first optical element is provided near a position where the plural color beams color-separated by the color separation means are focused in a sub scanning cross section by the imaging optical system and a second optical element is provided in an optical path after the first optical element and before the light receiving means, and PA1 in that a color image based on the plural color beams color-separated by the color separation means is focused through the first optical element and the second optical element on the surface of the light receiving means. PA1 in that the conjugate relation in the second optical element is an imaging relation of a reduction ratio; PA1 in that the first optical element is divided into at least three regions and elements in the three regions have mutually different element structures; PA1 in that the first optical element is a diffraction optical element; PA1 in that the second optical element is a diffraction optical element; PA1 in that the one-dimensional blazed diffraction grating is a reflection type one-dimensional blazed diffraction grating; PA1 in that the one-dimensional blazed diffraction grating is a transmission type one-dimensional blazed diffraction grating, and so on.
This means that the 3-line sensor 104 should be constructed by such asymmetric line spaces 116, 115 that the distances between the sensor lines of the monolithic 3-line sensor 104 are 0.171 mm on the + first-order beam (.lambda..sub.+1) side (R) (i.e., the distance 116 to the line 110) and 0.136 mm on the - first-order beam (.lambda..sub.-1) side (B) (i.e., the distance 115 to the line 108) with respect to the center line 109 (G). This realizes a reasonable color reading apparatus not necessitating the redundant line memories for interpolation described previously.
However, the color image reading apparatus using this one-dimensional blazed diffraction grating as an optical element for color separation has a problem that the resolution of reading in the sub scanning direction is degraded by broadening, which is to be called as fine color blur, on the surface of light receiving means, which occurs when each of the .+-. first-order diffracted light components separated and diffracted by the one-dimensional blazed diffraction grating includes a finite wavelength band.